Have a few questions in mind to ask the supervisor regarding his/her work, the lab, and the area the university is in. Also, be able to explain why you want to pursue a PhD and why research/their work interests you.

Keep a copy of your CV handy so you can refer to it during the interview. And as mentioned before do a practice run with a friend to work out any technical bugs. Skype interviews were way easier than phone interviews because you can see how the conversation is progressing, who is talking, facial expressions and what not.

Practice talking to the camera (making "eye contact") rather than looking at the video feed of the interviewers when you are talking. I found that my natural inclination is to look at the video feed and I have to try hard to remember to look at the camera. Also,for your own piece of mind, try to find a good angle for your camera- no one wants to look up your nose or at only the top half of your face. Good luck!